Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The cost and conversion efficiency of LEDs are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. Many high power applications require multiple LEDs to achieve the needed power levels, since individual LEDs are limited to a few watts. In addition, LEDs generate light in relatively narrow spectral bands. Hence, in applications requiring a light source of a particular color, the light from a number of LEDs with spectral emission in different optical bands is combined. Thus, the cost of many light sources based on LEDs is many times the cost of the individual LEDs.
The cost of an LED is increased by the need to package the LED containing dies prior to installing those dies on printed circuit boards and the like in the final product. The cost of conventional packaging that relies on wire bonds to connect the dies to an underlying circuit carrier represents a significant fraction of the cost of the final LED. Hence, “flip-chip” packages in which the LED die is modified such that the pads for powering the LED are on the opposite side of the die from the surface through which the light is emitted have been developed. The pads are constructed such that the die can be soldered directly to a pair of corresponding pads on a printed circuit board. Unfortunately, the techniques used to increase the conversion efficiency of LEDs are not easily implemented in the flip-chip architecture.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. Heat dissipation places a limit on the power level at which an LED operates. In addition, the LEDs must be mounted on structures that provide heat dissipation, which, in turn, further increases the cost of the light sources. Hence, if the conversion efficiency of an LED can be increased, the maximum amount of light that can be provided by a single LED can also be increased, and hence, reduce the number of LEDs needed for a given light source. In addition, the cost of operation of the LED is also inversely proportional to the conversion efficiency. Hence, there has been a great deal of work directed to improving the conversion efficiency of LEDs.
The spectral band generated by an LED, in general, depends on the materials from which the LED is made. LEDs commonly include an active layer of semiconductor material sandwiched between additional layers. For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers. These layers are typically deposited on a substrate such as sapphire. It should be noted that each of these layers typically includes a number of sub-layers.
Improvements in materials have led to improvements in the efficiency of light generated in the active layer. However, a significant fraction of the light generated in the active layer is lost. Most of this light is lost through absorption in the various layers used to construct the LED. This mode of light loss is aggravated by the trapping of much of the light within the LED structure. Light that is generated in the active layer must pass through the top layer or the substrate before exiting the LED. Since the active layer emits light in all directions, the light from the active region strikes the boundary between the outer layers of the LED at essentially all angles from 0 to 90 degrees relative to the normal direction at the boundary. Light that strikes the boundary at angles that are greater than the critical angle is totally reflected at the boundary. This light is redirected toward the other outer boundary and is likewise reflected back into the LED. As a result, the light is trapped within the LED until it strikes the end of the LED or is absorbed by the material in the LED. In the case of conventional GaN-based LEDs on sapphire substrates approximately 70 percent of the light emitted by the active layer is trapped between the sapphire substrate and the outer surface of the GaN.
Several techniques have been described to improve light extraction from LEDs, and hence, improve the light conversion efficiency of these devices. In one class of techniques, one of the outer surfaces of the LED is converted from a smooth planar surface to a rough surface. Some of the light that is reflected at the other surface will return to the rough surface at a location in which that light is now within the critical angle, and hence, escapes rather than being again reflected. The remainder of the light is reflected back toward the smooth surface and returns to the rough surface at a new location and at a range of angles, and hence, a portion of that light also escapes, and so on.
In a typical flip-chip LED, the light is emitted through the bottom surface of the LED, i.e., through the substrate on which the LED layers were deposited. Providing a rough surface at the substrate air boundary is not as effective in reducing the trapping of light within the high index GaN layers sandwiched between air and sapphire. Accordingly, the rough surface is normally provided on the top surface of the LED, unless it is applied to the bottom surface of the GaN by growing GaN on a surface-patterned sapphire substrate. However, in a flip-chip design, the top surface of the LED must also be a reflector. To convert the rough surface to a reflector, a layer of metal such as silver is applied to the surface. Unfortunately, the reflectivity of such a layer is substantially less than 100 percent due to surface plasmon effects caused by the underlying roughened layer.
To avoid this problem, prior art devices in which the substrate is removed to expose the bottom layer of epitaxially grown GaN are utilized. The LED is first bonded to a new substrate, the sapphire substrate is removed and the bottom surface is then roughened. However, this technique has its own problems. First, the substrate removal process is not as well developed as the other types of processing used to fabricate LEDs. Second, the process involves a number of additional steps that increase the cost of the LEDs. Finally, if during any period of time the LED wafer is without a substrate, the wafer is extremely fragile, and hence, subject to damage.